Superheavy nuclides / elements

The problem of the superheavy nuclides with the atomic numbers around 114 stimulated TC separation studies of their expected chemical homologs — the elements of groups 13 to 18 in the sixth period of the Mendeleev system. With the use of H2 as the reducing carrier gas, elements from Hg to Rn could be separated, mostly in their elemental state [83,84], An illustration is given in Fig. 1.20. This approach was used in searching (yet unsuccessful) for SHEs in a uranium target after it had been bombarded with Xe ions [85]. 

The Karlsruhe Chart of the Nuclides has this same basic structure but with the addition of all known radioactive nuclides. The heaviest stable element is bismuth (Z = S3, N = 126). The figure also shows the location of some high Z unstable nuclides - the major thorium (Z = 90) and uranium (Z = 92) nuclides. Theory has predicted that there could be stable nuclides, as yet unknown, called superheavy nuclides on an island of stability at about Z = 114, A = 184, well above the current known range. 

It appears that scientists have now been on the Island of Stability —for at least 30 seconds. In recent years, the island of stability has attracted nnclear scientists the way Mount Everest has attracted mountain climbers. Like mountain climbers, scientists have wanted to surmount their own tallest peak—discovery of the relatively stable, superheavy nuclide at the peak of stability that is surrounded by foothills comprised of less stable nuclides. Discovering the superheavy nuclide at the peak would also mean discovering a new element—a task that has had its allure since Lavoisier gave currency to the concept of elements in the late eighteenth century. 

Probing superheavy element space by " Ca-induced hot-fusion reactions is characterized by advancing beyond the = 162 deformed subshell closure toward nuclei that are spherical and tightly bound. The macroscopic-microscopic model characterizes the ground-states of nuclei with > 175 as having a prolate deformation parameter 62 < 0.1, making them nearly spherical [8, 58, 60]. At neutron numbers below N — 175, any cross section benefit of the " Ca-induced hot-fusion approach to the Island of Stability is expected to decrease, as the shell stabilization of the ground state of the compound nucleus decreases. If a coldfusion path to a particular superheavy nuclide is available, it is expected to be the better one however, very little experimental evidence of this is available. As an example, the attempt to produce Cn (Z — 112) in the UC Ca,4n) reaction was unsuccessful (cross section limit <0.6 pb) [8, 316], in contrast to its production in the ° Pb( °Zn,n) reaction (cross section 0.5 pb) [263, 271]. 

The decay chains of the " Ca-produced superheavy nuclides span an interesting region of nuclear structure in which near-spherical nuclei gradually become more deformed as the neutron number decreases [58, 60]. Unlike the cold-fusion superheavy nuclides, the decays of these nuclei probe the N — 162 subsheU closure and the transition region between A = 184 and A = 162 from the neutron-rich side. This results in a complicated stmcture in the a spectra of the lower-Z members of the decay chains arising from superheavy nuclides with unpaired nucleons [8]. There is a significant possibility of nuclear isomerism in these interesting nuclei, as has been reported for cold-fusion Ds isotopes (see Sect. 2.3). For a more comprehensive discussion of isomers and superheavy elements, see Nuclear Structure of Superheavy Elements . 

Early efforts to produce superheavy nuclides in " Ca-induced reactions suffered from a lack of sensitivity [338-344]. Expectations for the production cross sections for evaporation residues in this work were optimistic, and experiments at that time had no chance of observing nuclides produced at the level of single-digit pico-barns. Expectations for nuclide half-lives were similarly optimistic, and some of the work relied on radiochemical separations following irradiations of extended length. See Historical Reminiscences The Pioneering Years of Superheavy Element Research for more details about these historical experiments. 

Under this constraint, it is not possible to produce new superheavy nuclides at greater neutron excess by cold fusion, or by hot fusion with heavy-ion beams with lower atomic numbers than argon. This is because of the neutron richness of the overshoot isotopes, daughters of the multiple emission of relatively proton-rich a particles in the decays of the " Ca-induced evaporation residues. Nevertheless, both reaction types offer advantages in the production rates of the known isotopes of superheavy elements with Z = 106-108 that are of interest to the radiochemist. As examples Direct production of the long-lived hassium isotope Hs is possible in the cold-fusion irradiation of ° Pb with radioactive Fe. From Fig. 2, the cross... 

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

The Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia, has recently announced the observation of relatively long-lived isotopes of elements 108, 110, 112, 114, and 116 [63-66] confirming the over 30 years old theoretical prediction of an island of stability of spherical superheavy elements. Due to the half-lives of the observed isotopes in the range of seconds to minutes, chemical investigations of these heaviest elements in the Periodic Table appear now to be feasible. The chemistry of these elements should be extremely interesting due to the predicted dramatic influence of relativistic effects [67], In addition, the chemical identification of the newly discovered superheavy elements is highly desirable as the observed decay chains [63-66] cannot be linked to known nuclides which has been heavily criticized [68,69],... 

Direct searches for superheavy elements in the U+ U reaction were undertaken at the unilac by several groups. All these efforts remained without positive evidence. The data are summarized in Figure 13. The curve labeled chem [106] was obtained with off-line chemical separations [107] and an assay for a-and spontaneous fission activities here, the 10 picobam level was reached for half-lives between several days and years. Attempts to detect short-lived nuclides were less sensitive. The curve labeled gas holds for an on-line search [108] for components volatile at room temperature. wheel [106] refers to fission track detection in the unseparated product mixture deposited on a rotating catcher, rec [109] to implantation of recoil atoms in a surface barrier detector, and JET to on-line transport from target to detector with a gas jet [91,110],... 

Today, one century after Ernest Rutherford and Frederick Soddy postulated that in the radioactive decay one chemical element transmutes into a new one, we know of 112 chemical elements. The discoveries of elements 114 and 116 are currently waiting to be confirmed and experimentalists are embarking to discover new and heavier elements. Now where are superheavy elements located on a physicist s chart of nuclides and on the Periodic Table of the Elements - the most basic chart in chemistry ... 

The main fields of application of heavy ions are synthesis of new elements (superheavy elements), production of nuclides far away from the line of ji stability (exotic nuclides), investigation of nuclear matter at high densities, production of small holes of certain diameters in thin foils and irradiation of tumours in medicine. 

The islands of relative stability are shown in Fig. 14.9. The stability gap around mass number 4 = 216 is evident from this figure nuclides with half-lives > 1 s do not exist for = 216. The search for superheavy elements concentrates on the islands around Z = 108 and Z= 114. At neutron numbers A = 162 the nuclei should exhibit a high degree of deformation, whereas spheric nuclei are expected for N = 184. 

Herrmann G (2003) Historical reminiscences. In Schadel M (ed) The chemistry of the superheavy elements. Kluwer, Dordrecht, pp 291—316 Herrmann G, Trautmann N (1982) Rapid chemical methods for identification and study of short-lived nuclides. Ann Rev Nucl Part Sci 32 117 Hevesy G (1915) Uber den Austausch der Atome zwischen festen und fliissigen Phasen. Physik Z 16 52 Hevesy G (1923) Absorption and translocation of lead by plants, A contribution to the application of the method of radioactive indicators to the investigation of the change of substance in plants. Biochem J 17 439... 

O Figure 21.11 of Chap. 21, Superheavy Elements, gives a clearer picture of the nuclides beyond A 200. There is an abrupt absence of nuclides with moderate much less long half-lives between ° Pb and Th. This is due to shell effects that are not included in the semiempirical equation. Of course shell effects are crucial for stabilizing the several islands of stability among heavy elements, which include the parents of the natural decay series as well as surprisingly stable isotopes of elements well beyond uranium. 

The transactinides are the doorway to the superheavy elements. The region of shell stabilization starts here (O Fig. 19.1) without which the chart of the nuclides would end around seaborgium. For this reason, these elements are sometimes also referred to as "superheavy nuclei. In this chapter the conventional definition will be adopted when referring to the superheavy elements spherical nuclei that he around the next double shell closure above Pb. 

Nuclear Structure of the Transactinide Nuclides 19.4.1 Superheavy Elements The Limits of Stability... 

Fermi irradiated uranium with slow neutrons, and observed a variety of radioactivities that he tentatively identified as being transuranium elements [1]. We now know that these radioactive species were the products of the fission of the in the sample. Study of the chemical properties of these new nuclides led to the subsequent discovery of fission in 1939 [2, 3], Explanation of the fission process was closely connected to the creation of the liquid-drop model [4—6], in which the nucleus is treated like an incompressible charged fluid with surface tension. See Nuclear Structure of Superheavy Elements for more information on nuclear structure and the stability of the heaviest nuclides. 

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